1. Field of the Invention
The present invention relates to a semiconductor device, and more particularly relates to a semiconductor device with a termination resistance adjusting circuit for adjusting a termination resistor.
2. Description of the Related Art
A technique is conventionally known in which a termination resistor is built in a semiconductor device in order to attain cost-down of a transmission system and reduction of a substrate mounting area. When the termination resistor built in the semiconductor device is not adjusted, there may be a high possibility that the resistance of the termination resistor has a wide distribution due to manufacturing variation of the termination resistor. If the resistance of the termination resistor is out of a desired range, signal reflection is caused to deteriorate transmission quality, which results in the decrease in a production yield. Also, the resistance of the termination resistor is a factor to determine an output signal voltage of a transmitting circuit in the transmission system. Therefore, stabilization of both the resistance of the termination resistor and the output signal voltage is required.
Conventional techniques are known in Japanese Laid Open Patent Application (JP-P2003-298395A and JP-P2004-32721A: first and second conventional examples), in which a resistance adjusting circuit is built in a semiconductor device to operate a circuit adequately even if the resistance of a termination resistor is out of a desired range.
FIG. 1 is a block diagram showing the configuration of a termination resistance adjusting circuit 100 in the first conventional example. In the first conventional example, a stable reference current Iref is generated based on reference voltages VrefH and VrefL and an external resistance 119 and is applied to a replica resistor 130. Voltages Va and Vb generated at that time, and the reference voltages VrefH and VrefL are compared by a control voltage generator 120. Thus, the control voltage generator 120 recognizes the difference between the replica resistor 130 and the external resistance 119 from the comparison result and carries out an adjustment so that the resistance of the replica resistor 130 and that of the external resistance 119 are coincident with each other.
Also, FIG. 2 is a block diagram showing the configuration of an impedance variable circuit 200 in the second conventional example. In the impedance variable circuit shown in FIG. 2, a synthetic resistance of resistors (201 to 209) is varied by controlling switches (SW1 to SW9) and is used as a termination resistor.
FIG. 3 is a circuit diagram showing a specific circuit configuration of a termination resistance adjusting circuit 300, which is constituted from the above-mentioned termination resistance adjusting circuit 100 and the impedance variable circuit 200. As shown in FIG. 3, the termination resistance adjusting circuit 300 is composed of a termination resistor generator 101, a transmitting circuit 102, a first reference current generator 104, a second reference current generator 105 and a termination resistance controller 106. In the termination resistance adjusting circuit 300, the first reference current generator 104 generates a stable reference current Iref1 based on a reference voltage Vref and an external resistance 109, and applies the reference current Iref1 to a replica resistor 130 of the termination resistance controller 106. Also, the second reference current generator 105 generates a stable reference current Iref4 based on a reference voltage Vref and an internal resistance 108, and applies the reference current Iref4 to a replica resistor 131 of the termination resistance controller 106. The termination resistance controller 106 compares a voltage V1 generated based on the current Iref1 and the resistor 130 and a voltage V2 generated based on the current Iref4 and the resistor 131, and recognizes the difference between the internal resistance 108 and the external resistance 109 from the comparing result. Then, the termination resistance controller 106 outputs a control signal Vcont to the termination resistor generator 101 based on the comparing result. The termination resistor generator 101 generates a resistance through separation and synthesis of resistors in response to the control signal Vcont, such that a precision of the internal resistance is coincident with that of the external resistance.
When the termination resistor is assumed to be a resistor value R150 and the reference current is assumed to be a reference current Iref2, a transmitting circuit output voltage Vo is represented by:Vo=R150*Iref2   (1)
In the conventional termination resistance adjusting circuit 300 shown in FIG. 3, the voltages V1 and V2 are compared. The voltage V2 is generated when the current Iref4 is applied to the resistor 131. The current Iref4 is generated based on a reference voltage Vref and the internal resistance 108. The reference voltage Vref is supplied from a band gap power supply circuit or the like, in which an output voltage variation caused due to external factors such as a temperature variation, a power source voltage variation and the like is small.
The voltage V1 is generated when the current Iref1 is applied to the resistor 130. The current Iref1 is generated based on the reference voltage Vref and the external resistance 109 which is more stable than the internal resistance in an absolute precision. The reference voltage Vref is supplied from the above-mentioned band gap power supply circuit or the like.
Here, when the internal resistances 108 and 131 are assumed to be R108 and R131, respectively, and the external resistance 109 and the internal resistance 130 are similarly assumed to be R109 and R130, respectively, the voltages V1 and V2 are represented by the following equations.V2=(Vref/R108)*R131   (2)V1=(Vref/R109)*R130   (3)In this case, since the internal resistances 131 and 108 have the same structure, the relative precision is insured. Therefore, the item of “R131/R108” in the above equation (2) has a constant value. Thus, the voltage V2 is the stable voltage similar to the reference voltage Vref.
Also, since the external resistance 109 has an extremely high precision as compared with the internal resistance, the item of “Vref/R109” in the above equation (3) can be regarded as a constant value. Thus, the voltage V1 is a value proportional to the internal resistance 130. FIG. 4 shows the above relation. When the voltages V1 and V2 are compared, if the voltage V1 is determined to be excessively higher than the voltage V2 (namely, the internal resistance is excessively high), the control signal Vcont is outputted to adjust the resistance R150 to a low value. Consequently, the precision of the termination resistor 150 after the adjustment is similar to that of the external resistance 109.
However, the actual adjustment is carried out in a step manner of a definite range. Thus, even in the ideally adjusted state, the resistances before and after the adjustment are discontinuous, which brings about an error depending on the adjustment resolution of the termination resistor 150. In particular, it could be understood that the maximum error (ERR) is generated in the vicinity of the adjustment.
Under the assumption that the termination resistor has been adjusted, a fixed current Iref2 is generated based on the stable power voltage Vref and the external resistance 109 and applied to the transmitting circuit 102. Thus, the transmitting circuit output voltage Vo is a function of the termination resistor 150, as represented by the following equation (4).Vo=Iref2*R150   (4)
FIG. 5 is a diagram showing the waveform of an output signal outputted from the termination resistance adjusting circuit 300. With reference to the waveform shown in FIG. 5, the voltage error +ERR, −ERR remains in the output signal waveform due to the adjustment error of the termination resistor 150. The voltage error +ERR, −ERR sometimes causes poor measurement reproducibility or a large deviation of the output voltage of the transmitting circuit 102.
In order to avoid these problems, a method is known in which the adjustment resolution is made higher. However, if such a method is employed, the higher precision of the termination resistance controller 106 is required, which leads to a larger circuit scale. Also, since the number of switching circuits in the termination resistor generator 101 is increased, a capacitive load becomes greater, which restricts a frequency band.
The termination resistance adjusting circuit is desired in which the adjustment error to the termination resistor has no influence on the output voltage, without the increase in the circuit scale and the limitation on the frequency band.